LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 17

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Low-Ripple Burst Mode and Pulse-Skipping Mode
The LT3690 is capable of operating in either low ripple
Burst Mode operation or pulse-skipping mode, which is
selected using the SYNC pin. See the Synchronization and
Mode section for details.
To enhance efficiency at light loads, the LT3690 can be
operated in low ripple Burst Mode operation that keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3690 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output capacitor is delivers output power to the load.
Because the LT3690 delivers power to the output with
single, low current pulses, the output ripple stays below
15mV for a typical application. In addition, V
quiescent currents are reduced to 35µA and 70µA (typical),
respectively, during the sleep time. As the load current
decreases towards a no-load condition, the percentage
of time that the LT3690 operates in sleep mode increases
and the average input current is greatly reduced, resulting
in high efficiency even at very low loads (see Figure 3).
At higher output loads (above about 385mA at V
for the front page application) the LT3690 will run at the
frequency programmed by the R
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and does
not disturb the output voltage.
10mV/DIV
0.5A/DIV
5V/DIV
V
V
OUT
SW
I
L
V
FRONT PAGE APPLICATION
IN
Figure 3. Burst Mode Operation
= 12V: I
LOAD
= 20mA
5µs/DIV
T
resistor, and operate in
3690 F03
IN
and BIAS
IN
= 12V
If low quiescent current is not required, the LT3690 can
operate in pulse-skipping mode. The benefit of this mode
is that the LT3690 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode operation. The front page application circuit
will switch at full frequency at output loads higher than
about 64mA at V
Low Side Switch Considerations
The operation of the internal low side switch is optimized
for reliable, high efficiency operation. The low side switch
is connected in parallel with a catch diode. When the top
side switch turns off, the inductor current pulls the SW
pin low, and forward biases the internal catch diode. In
order to prevent shoot through currents, the internal low
side switch only turns on after detecting the SW pin going
low. Once the low side switch turns on, the voltage drop
between SW and GND is very small, minimizing power
loss and improving efficiency. At the end of the switching
cycle, the low side switch turns off, and after a delay, the
top side switch can turn on again. The switching sequence
is shown in Figure 4.
The overload comparator monitors the current flowing
through the low side switch and helps protect the circuit.
This comparator delays switching if the low side switch
current goes higher than 5A (typical) during a fault con-
dition such as a shorted output with high input voltage.
2V/DIV
Figure 4. Switching Sequence of High Side,
Catch Diode and Low Side Switch
V
SW
0V
IN
= 12V.
200ns/DIV
V
V
L = 3.3µH
IN
OUT
= 12V
= 3.3V
3690 F04
LT3690
17
3690f